V I S H A Y I N T E R T E C H N O L O G Y, I N C .
LOW VALUE POWER METAL STRIP® RESISTORS
Intelligent Li-ion Rechargeable Battery
Intelligent Li-ion Rechargeable Batteries
Utilize Current Sensing Resistors with Ultra
Low Ohm, Low TCR, Tight Tolerance, High
Withstanding Surge Current and Low Thermal
EMF
In the field of portable electronic equipment such
as Notebook PCs, mobile telephones and
camcorders, the demand for Intelligent Li-ion
rechargeable batteries has been showing rapid
growth because their high energy density
(approximately 1.5 times in volume and 2.0
times by weight compared to Ni-MH batteries),
high output voltage (+ 3 V to + 4 V), and low selfdischarge rate (10% to 12%/month).
However, to assure the performance of Intelligent
Li-ion rechargeable batteries, three inherent
difficulties must be taken into account. First,
heavy current discharge will cause degraded
performance, i.e., available energy will drop
down to 90% and sometimes 50% of nominal
energy. Second, high internal impedance causes
a potential drop which makes it difficult to utilize
the expected potential energy at high efficiency.
Third, required constant-current and constantvoltage charging is difficult to control.
To solve the above difficulties, various Intelligent
Electronic Charger and Monitoring Circuits have
been developed that utilize resistive components.
(See Appendix.)
For the precise determination of charge and
discharge activities of the battery, very stable
and accurate sense resistors with the following
characteristics are required:
•
w w w . v i s h a y. c o m
Since the maximum normal operating current
is three to four amps, in general, the
maximum ∆R due to self-heating must be as
low as 1000ppm across the ambient
temperature range of 0 to + 40˚C. Thus, a
low TCR resistor for current sensing is
required. See Chart 1 for TCR comparison.
•
Low Thermal EMF
During the standby mode, a Notebook PC
requires 50 to 100 milli-amps to operate its
D-RAM and CPU, and a Handycam requires
5 milli-amps to operate its small memory.
Therefore, in the standby mode, the thermal
EMF of the sense resistor must be low
compared to the terminal voltage generated
by the current output.
•
Resistance Value
For minimizing the power loss and charging
time, and maximizing the efficiency of the
potential energy of the Intelligent Li-ion
rechargeable batteries to the connected
equipment, the sense resistor must be as
close to zero as possible. However, this
ideal condition is unrealizable since the
limited resolution of the microcomputer
requires a certain level of voltage between
the terminals of the sense resistor.
The key factors to determine the resolution
of the microcomputer are its semiconductor
noise and offset voltage. Typical resistance
values utilized in various micro-computers
are 100, 50, 20 and 10 milli-ohms.
Tight Tolerance
The Vishay Dale® WSL-0805, WSL-1206,
WSL-2010, WSL-2512, WSL-0805-18, WSL1206-18, WSL-2010-18, WSR2 and WSR3
can provide extremely low values down to 1
milli-ohm with one resistor. This saves
mounting costs and space, and improves
the MTBF of Intelligent Li-ion rechargeable
batteries.
To maintain the total accuracy of the Intelligent
Li-ion rechargeable batteries as close as
+ 5%, - 0% of the total available power
capacity, the tolerance of the sense resistor
must be ± 1% or tighter.
In comparison, with conventional cermet
chips, four to six chips are needed to achieve
very low ohmic values. And with conventional
thin film chips, at least two chips are needed
due to their low power capability.
Very Low Ohmic Value
For minimizing energy loss, the required ohmic
value is below 100 milli-ohms.
•
Low TCR
VSD-AP0002-0301
APPLICATION NOTE
Sense Resistor-General Application
•
Application Note WSL and WSR for Li-ion Batteries
Vishay Dale
Appendix I
Appendix II
Microcomputers for Intelligent Li-ion Rechargeable
batteries
Intelligent Charger Circuits
There are two basic Intelligent Charger Circuits:
Microcomputers have various functions to maintain an accurate
record of the available capacity of Intelligent Li-ion rechargeable
batteries.
• To monitor a voltage drop across a sense resistor, connected
in a series between the negative battery terminal and ground,
to determine the charge and discharge activity of the battery.
• To apply accurate compensations for battery temperature
and rate of charge or discharge and self-discharge calculations
to provide available capacity information across a wide range
of operating conditions.
• To automatically recalibrate the battery capacity or to learn
the capacity in the course of a discharge cycle from full to
empty.
The sequence of battery charging is first, constant current
charging until the terminal voltage reaches 4.1V or 4.2V. Then
second, switched to constant voltage charging mode until full
charge is achieved. Immediately after full charge is achieved,
the sequence must be stopped.
• Active Potential – Conventional Type
When the Intelligent Li-ion rechargeable batteries are
removed from the equipment, the potential between the
terminals is still active (Figure 1). In the case of short circuit by
misuse, they could be seriously damaged due to an
abnormally high discharge current. Therefore, Intelligent Liion rechargeable batteries require protective resistors or fuse
resistors to cut off the abnormally high current. This
inactivates the circuit.
FIGURE 1: Active Potential Type (Conventional)
MICRO-COMPUTER
POWER MOSFET
CHARGE CURRENT
CUT-OFF
+
-
SENSING RESISTOR
When removed from the equipment, the potential between the terminals
is still active. Thus, to protect the charging circuit, fuse resistors which
burn out at high currents are used.
High Ohmic Values Increase The Time For
Charging And Causes Large Power Loss
CONSTANT
VOLT
CHARGING
CONSTANT
CURRENT
CHARGING
FULL
CHARGE
R1
V
I (R2)
• Non-Active Potential – Advanced Type
When the Intelligent Li-ion rechargeable batteries are
removed from the equipment, the potential between the
terminals is zero (Figure 2). Thus, they do not have a chance
of electrical short circuit.
FIGURE 2: Non-Active Potential Type (Advanced)
R2
POWER MOSFET
CHARGE CURRENT
I (R1)
V (R1)
DISCHARGE CURRENT
CUT-OFF
MICRO-COMPUTER
+
-
V (R2)
SENSING RESISTOR
TIME: APPROXIMATELY 30 MINUTES
i.e. R1 = 10 Milli-ohms; R2 = 50 Milli-ohms
www.vishay.com
2
When removed from the equipment, the potential between the terminals
is still zero. Thus, the Vishay Dale® Power Metal Strip® Resistor is used.
For technical questions, contact ww2bresistors@vishay.com
Application Note WSL and WSR for Li-ion Batteries
Vishay Dale
CHART 1
Resistance Temperature
Coefficient (ppm/°C)
POWER METAL STRIP - TCR PERFORMANCE
Resistance Value (mΩ)
NOTES:
For technical questions, contact ww2bresistors@vishay.com
www.vishay.com
3
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